'Create table if not exists as' breaks SPI_execute

diff --git a/src/backend/executor/spi.c b/src/backend/executor/spi.c
index fd94179..49ebce4 100644
--- a/src/backend/executor/spi.c
+++ b/src/backend/executor/spi.c
@@ -2217,15 +2217,21 @@ _SPI_execute_plan(SPIPlanPtr plan, ParamListInfo paramLI,
 				 */
 				if (IsA(stmt, CreateTableAsStmt))
 				{
-					Assert(strncmp(completionTag, "SELECT ", 7) == 0);
-					_SPI_current->processed = pg_strtouint64(completionTag + 7,
+					CreateTableAsStmt *ctastmt = (CreateTableAsStmt *) stmt;
+
+					if (strncmp(completionTag, "SELECT ", 7) == 0)
+						_SPI_current->processed = pg_strtouint64(completionTag + 7,
 															 NULL, 10);
+					else if (*completionTag == '\0' && ctastmt->if_not_exists)
+						_SPI_current->processed = 0;
+					else
+						Assert(false);
 
 					/*
 					 * For historical reasons, if CREATE TABLE AS was spelled
 					 * as SELECT INTO, return a special return code.
 					 */
-					if (((CreateTableAsStmt *) stmt)->is_select_into)
+					if (ctastmt->is_select_into)
 						res = SPI_OK_SELINTO;
 				}
 				else if (IsA(stmt, CopyStmt))